Which player in your computer are you using?
Normally, you select the folder that the movie is in. For example, you have a folder named Harry Potter and the Chamber of Secrets, which contains two other folders, named Video_TS and Audio_TS. With VLC or Media Player Classic Home Cinema, you can select this main folder with the name of the movie. Or you can select the Video_TS folder.
Some players will let you select the the Video_TS.IFO file to start the movie. It is located in the Video_TS folder of all DVD-videos. But it normally isn't necessary to go this far in to play the movie.